Grown and Characterization of Zno Aligned Nanorod Arrays for Sensor Applications

energies Article Grown and Characterization of ZnO Aligned Nanorod Arrays for Sensor Applications Arkady N. Redkin, Eugene E. Yakimov, Maria V. Evstafieva and Eugene B. Yakimov * Institute of Microelectronics Technology RAS, 6 Academician Ossipyan str., 142432 Chernogolovka, Russia; [email protected] (A.N.R.); [email protected] (E.E.Y.); [email protected] (M.V.E.) * Correspondence: [email protected] Abstract: ZnO nanorods are promising materials for many applications, in particular for UV de- tectors. In the present paper, the properties of high crystal quality individual ZnO nanorods and nanorod arrays grown by the self-catalytic CVD method have been investigated to assess their possible applicationsfor UV photodetectors. X-ray diffraction, Raman spectroscopy and cathodoluminescence investigations demonstrate the high quality of nanorods. The nanorod resistivity and carrier concentration in dark is estimated. The transient photocurrent response of both as grown and annealed at 550 ◦C nanorod array under UV illumination pulses is studied. It is shown that annealing increases the sensitivity and decreases the responsivity that is explained by oxygen out-diffusion and the formation of near surface layer enriched with oxygen vacancies. Oxygen vacancy formation due to annealing is confirmed by an increase of green emission band intensity. Keywords: ZnO nanorod; self-catalytic CVD method; photoresponse; cathodoluminescence Citation: Redkin, A.N.; Yakimov, E.E.; Evstafieva, M.V.; Yakimov, E.B. Grown and Characterization of ZnO 1. Introduction Aligned Nanorod Arrays for Sensor Zinc oxide, a well-known direct bandgap II–VI semiconductor, is a material with Applications. Energies 2021, 14, 3750. large exciton binding energy (60 meV) and a wide bandgap (Eg ~ 3.37 eV) [1], suitable for https://doi.org/10.3390/en14133750 short wavelength optoelectronic applications [2]. It is a promising material for fabricating photonic [2,3], optical [4–6], electronic [7,8] and photovoltaic devices [9,10]. Additionally, Academic Editors: Wilson ZnO is transparent to visible light and can be made highly conductive by doping [11,12]. Merchan-Merchan, Alexei Saveliev The non-centrosymmetry in ZnO wurtzite structure and the polarity developed along the c and Peter Foot axis make this material piezoelectric, which, in combination with its large electromechanical coupling, results in strong piezoelectric and pyroelectric properties useful in piezoelectric Received: 13 May 2021 sensors [13,14] and nanogenerators [15,16]. The ferromagnetism of doped [17] and undoped Accepted: 17 June 2021 Published: 22 June 2021 ZnO [18] makes it a promising material for spintronics. Additional useful advantages of zinc oxide include low toxicity, chemical stability, electrochemical activity, making it a Publisher’s Note: MDPI stays neutral promising material for biosensors, and biomedical applications [19,20]. 10 2 with regard to jurisdictional claims in In the strict sense, zinc is not a transition metal as it has d s electronic configuration published maps and institutional affil- with a completely filled d-shell and can be classified as post-transition metal. However, iations. it exhibits many properties similar to those of transition metals and sometimes it is often convenient to include this element in a discussion of the transition elements. In zinc oxide, s electrons are strongly pulled by oxygen and consequently the structural, physical and chemical properties are mostly determined by the d electrons, in certain sense similar to transition metal oxides. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Miniaturization in electronics and development of different photonic devices requires This article is an open access article a new generation of cheap, energy-efficient, nano- or submicron sized semiconductor distributed under the terms and lasers. One-dimensional (1D) ZnO nanocrystals have a perfect structure and a developed conditions of the Creative Commons surface, which gives them certain advantages when used in the abovementioned practical Attribution (CC BY) license (https:// applications [5,16]. In this regard, the ordered arrays of zinc oxide nanorods (NR) obtained creativecommons.org/licenses/by/ by various methods are of great interest to researchers [21,22]. To obtain ZnO nanorods, 4.0/). various methods were used, such as hydrothermal synthesis, solvothermal method, sol-gel Energies 2021, 14, 3750. https://doi.org/10.3390/en14133750 https://www.mdpi.com/journal/energies Energies 2021, 14, 3750 2 of 9 method, chemical vapor deposition, organometallic chemical vapor deposition, magnetron sputtering, laser ablation, etc. [2,21,23]. Most of these processes are carried out at relatively low temperatures that lead to a decrease in the cost of the structures but increases inthe number of defects. Therefore, despite the abundance of methods, the synthesis of high- quality ZnO NRs with a perfect structure is not an easy task. The properties of ZnO NRs can vary greatly, even within the same method of synthesis. In [24] the original self- catalytic CVD procedure for growing zinc oxide nanorod arrays was developed. One of the advantages of this method is the possibility to deposit ordered arrays of high-quality single- crystal ZnO nanorods both on silicon substrates of various orientations and on inexpensive transparent glass substrates [25], which makes it attractive for practical applications. The method was developed to grown high quality nanorods for use as a laser medium. It was interesting to study the performance of such nanorods as UV sensors. In present work, arrays of ZnO nanorods were grown by the self-catalytic CVD procedure. The properties of nanorods were investigated to assess their possible applications for UV photodetectors. The performance of the UV sensors depends both on the state of the zinc oxide surface and on the concentration of intrinsic defects, which strongly depends on synthesis conditions and/or by post-synthesis treatment. 2. Materials and Methods The NR arrays were synthesized in a flow type two-zone quartz reactor in accordance with the previously developed method [24–26]. A charge of granulated high-purity (99.99%) zinc was placed in the first (evaporation) zone, and substrates were placed in the second (synthesis) zone. Si {100}, fused silica and glass wafers were used as substrates. Synthesis was carried out at a reduced pressure under the conditions of continuous evacuation. The pressure in the reactor was kept at a level of 103 Pa. The process was carried out at temperatures of 610 ◦C and 550 ◦C in the evaporation and synthesis zones, respectively. During the process, zinc was evaporated in the first zone, from which zinc vapor arrived at the second cooler zone, where it was partially condensed, forming an array of zinc nanodrops on the substrates, which are sufficiently uniform in size. These Zn drops serve as a catalyst, in contrast to the usual CVD process in which a noble metal (gold) is used as a catalyst. Further, when a high-purity oxygen–argon mixture (15% O2) enters the growth zone, its chemical interaction with liquid zinc occurs. Due to the reaction of zinc with oxygen, zinc oxide nanocrystals were deposited onto the substrates under the zinc droplets. Further, when oxygen enters the growth zone, its chemical interaction with liquid zinc occurs. The formed oxide is dissolved in a drop of zinc to form a supersaturated solution, from which solid ZnO crystallizes at the metal/substrate interface. The gas mixture was supplied to the reactor with a rate of 6 L/h. The synthesis was carried out for 20–30 min with the zinc consumption of 12–15 g/h. The diameter of the growing NRs corresponds to the diameter of the liquid zinc drop. Using this procedure, high-quality ZnO NR arrays were grown, the diameter and length of which can be varied by changing the synthesis conditions (duration, reagent consumption, etc.). The above procedure allows us to synthesize arrays of well faceted vertically aligned single-crystal NRs with a 150–250 nm diameter and up to 10 µm length. To obtain individual nanorods they were separated from the substrate by ultrasonic treatment and transferred on a silicon substrate. The cathodoluminescence (CL) investigations of arrays and individual nanorods of different size and geometry were carried out in the JSM 6490 (JEOL) SEM equipped with the MonoCL3 system (Gatan) and with the Hamamatsu photomultiplier as a detector. The investigations were carried out in the temperature range from 90 to 300 K. In the most cases the beam energy in the range from 10 to 20 kV and beam current of 0.1–1 nA were used for the CL measurements. The crystallinity of NRs was examined by X-ray diffractometry (Q–2Q) in the scheme of a two-crystal diffractometer on a laboratory BRUKER D8 Discover X-ray source with a rotating copper anode (CuKα radiation, λ = 1.54 Å).´ The Raman spectra of the samples Energies 2021, 14, 3750 3 of 10 Energies 2021, 14, 3750 The crystallinity of NRs was examined by X-ray diffractometry (Θ-2Θ) in the scheme3 of 9 of a two-crystal diffractometer on a laboratory BRUKER D8 Discover X-ray source with a rotating copper anode (CuKα radiation, λ=1.54 Ǻ). The Raman spectra of the samples werewere studied using aa BrukerBruker SenteraSentera RAMAN RAMAN microscope microscope under under the the excitation excitation by by a 532 a 532 nm nmsolid-state solid-state laser. laser. ToTo study study the the photoresponse of of nanorod arra arraysys vertically oriented oriented nanorods nanorods were were growngrown on quartzquartz substratessubstrates coatedcoated with with a thina thin polycrystalline polycrystalline ZnO ZnO film. film. The The diameter diameter and anddensity density of nanorods of nanorods as estimated as estimated by the by SEM the was SEM of 150was nm of and150 4nm× 10 and8 cm 4− ×2 ,10 respectively.8 cm−2, respectively.For current For measurements current measurements a quartz plate a quartz with twoplate indium with two contacts indium deposited contacts in deposited the form inof the strips form with of astrips length with of2 a mm length and of a distance2 mm and of 3a mmdistance between of 3 themmm between was pressed them to was the pressednanorod to arrays.

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